1. Field of the Invention
The present invention relates generally to antennas. More specifically, the present invention relates to a fan shaped conical antenna that is suitable for use in a direction finding array used in a direction finding system. Instead of using a full circular cone, the antenna includes only a sector of a complete circular cone. The antenna is used in a direction finding array of antennas that operates in the VHF/UHF/SHF bands.
2. Description of the Related Art
Conical antennas, which include a single inverted cone over a ground plane, and biconical antennas, which include a pair of cones oriented with their apexes pointing toward each other are used as broadband antennas for various applications, including direction finding. FIG. 1 is a schematic diagram illustrating two such antennas. A biconical antenna 100 includes a top inverted cone 101a and a bottom cone 101b. An electronic coupler 102 provides a connection to a feeding circuit (not shown) that provides an electrical signal that feeds the antenna. It should be noted that the antenna is symmetric about the cone axis and that each of the cones is a full cone, spanning 360.degree.. Similarly, a single cone antenna 110 includes a single antenna cone 111 that also spans 360.degree. and is symmetric about the cone axis. Single antenna cone 111 is connected to an electronic coupler 114 that provides a connection to a feeding circuit (not shown) that provides an electrical signal that feeds the antenna. The single cone antenna is located over a ground plane 112.
Antennas such as the ones shown above may be included in an array of antennas used for direction finding. Direction finding antenna arrays determine direction by comparing the phase or strength of signals received at different antennas. According to the principle of reciprocity, signals may likewise be sent from an array in a particular direction by altering the phase or strength of the electric signals feeding each of the antennas. Hereinafter, antenna surfaces such as top inverted cone 101a and bottom cone 101b are referred to as radiators and it should be understood that the antenna surfaces may be used to either radiate or receive a signal.
Because the conical antennas shown in FIG. 1 are symmetric about the cone axis, the radiation pattern from the antenna is omniazimuthal or isotropic with respect to the azimuth angle. As a result, the radiation patterns from such antennas tend to interfere with each other when a group of such antennas are included in an array. This complicates direction finding and may reduce the accuracy that may be achieved. Furthermore, each of the antennas take up a relatively large amount of space and must be spaced apart in an array to ensure that they do not physically interfere with each other.
What is needed is a direction finding antenna and a direction finding array made up of broadband antennas that do not radiate isotropically with respect to the azimuth angle and which occupy less physical space than the full conical antennas shown in FIG. 1.